Conventionally, to decrease the size of electromechanical systems while guaranteeing a good sensitivity of measurements, it is advantageous to combine micro-electromechanical elements and nano-electromechanical elements. Such electromechanical are now known as M&NEMSs for “Micro- and Nano-ElectroMechanical Systems”.
Such M&NEMSs comprise force sensors, such as accelerometers, gyrometers, or also magnetometers. Such force sensors typically come in the form of devices comprising a mobile mass mechanically held by deformable elements such as springs. The mobile mass is further mechanically coupled to deformable structures such as measurement beams used to measure displacements of the mass. The measurement beams may for example be strain gauges or also resonators. The mass-beam assembly is held in suspension above a recess.
For example, in the case of an accelerometer, during a displacement of the sensor, an inertia force applies to the mobile mass and induces strain on the measurement beam. Conventionally, in the case of a resonator-type measurement beam, the strain applied by the mass induces a variation of the resonator frequency, and in the case of a measurement beam of variable-resistance type, the strain applied by the mass induces a variation of the electric resistance. The acceleration can be deduced from these variations.
It should thus be understood that it is advantageous to combine a mobile mass of micrometer-range thickness and a measurement beam of nanometer-range thickness. In particular, a significant mass of the mobile element enables to maximize the inertia force and thus to induce sufficient strain for the measurement beam. Further, by preferring a beam of small thickness, the strain applied by the mass to the beam is maximized. Such a layout thus also has the advantage of increasing the sensitivity of the force sensor.
Document EP 1 840 582 discloses such a force sensor, that is, a sensor where the mobile mass has a thickness greater than that of the beam, and further provides a method of manufacturing such a sensor based on a SOI (“Silicon On Insulator”) technology.
According to the first manufacturing method described in EP 1 840 582, the strain gauge is first etched in a surface layer of an SOI substrate, and then covered with a protection. A silicon epitaxy is then carried out on this surface layer to obtain a layer of desired thickness for the forming of the proof body. However, the epitaxial growth technique is complicated and expensive to implement and does not provide very large silicon layer thicknesses. Due to this limit, it is difficult to obtain an optimal sizing of the proof body, and thus of the mass thereof, to maximize the strain applied to the gauge.
According to the second manufacturing method described in EP 1 840 582, the mobile mass is first etched in an SOI substrate. A polysilicon layer of nanometer-range thickness is then deposited for the forming of the strain gauge. However, the small thickness of the polysilicon layers is still difficult to control, and their mechanical and electric properties are not as good as those of a single-crystal silicon layer. Further, the deposition of such a thin layer may be submitted to strain, such as deformations capable of affecting the gauge performance. It is thus difficult, with this method, to obtain a gauge having mechanical and electric features which optimize the sensor sensitivity.
Another solution may comprise using two different SOI substrates to separately form the mobile mass and the gauge, and then sealing the two substrates together. However, a misalignment of the different elements, particularly between the mobile mass, the gauge, and the recess, may occur during the sealing, thus increasing the risk of altering the general sensitivity of the sensor.